Merge from lp:~jakub/helenos/vfs-2.5-cherrypick

Merge from lp:~jakub/helenos/vfs-2.5-cherrypick

This merge cherry-picks some of the changesets from Jiri Zarevucky's:

  lp:~zarevucky-jiri/helenos/vfs-2.5

and then continues independently, yet sometime in a similar vein.

Roughly speaking, Jiri's branch is merged entirely up to its revision
1926 and then cherry-picked on and off until its revision 1965. Among
these changes are:

* relativization of the API,
* client-side roots,
* server-side mounts,
* inbox for passing arbitrary files from parent to child,
* some streamlining and cleanup.

Additional changes include:

* addressing issues introduced by the above changes,
* client-side I/O cursors (file positions),
* all HelenOS file system APIs begin with the vfs_ prefix and can be
  used after including vfs/vfs.h,
* removal of some POSIX-ish headers and definitions,
* additional cleanup.